Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser systems at the transmitter. These laser systems are designed or actively tuned to operate at different wavelengths. When their emissions are combined in an optical fiber, the resulting WDM optical signal has a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in semiconductor amplifier systems or gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman amplification scheme. At the receiving end, the channels are usually separated from each other using, for example, thin film filter systems to thereby enable detection by separate detectors, such as photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single data channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data in WDM systems is typically less than comparative non-multiplexed systems. This is because optical amplification systems required along the link are amortized across all of the separate wavelength channels transmitted in the fiber. With non-multiplexed systems, each channel/fiber would require its own amplification system. As an additional functionality, different wavelengths in WDM optical networks can also be used for wavelength routing of optical signals. This is sometimes referred to as metroWDM.
Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, optical components are required to combine or multiplex, the channels into, and separate, or demultiplex, the channels from, the WDM optical signal. Moreover, there is the danger of channel drift, where the channels lose their spectral separation and thus overlap each other. This interferes with channel demodulation at the receiving end.
Minimally, the optical signal generators, e.g. the semiconductor laser systems that generate each of the optical signals corresponding to the optical channels for a fiber link, must have some provision for wavelength or frequency control. Generated and transmitted optical channel wavelength should be controlled to within a fraction of the channel wavelength spacing. Especially in systems with center-to-center channel wavelength spacings of less than 1 nanometer (nm), the optical signal generator must have a precisely controlled carrier wavelength. Any transmitted wavelength wander impairs the demodulation of the wandering signal at the far end receiver, since the wavelength is now different from that expected by the corresponding optical demultiplexer and signal detector. The wandering wavelength signal can have decreased power in its intended receiver; it can also interfere with and impair the demodulation of spectrally adjacent channels when their spectrums overlap each other.
Frequency/wavelength lockers are typically used in semiconductor based laser systems to detect the wavelength of operation of the laser relative to the intended ITU grid channel center, for example, and then generate a feedback signal that can be used as a tuning signal to control the laser's wavelength of operation. Typically, lockers are used to stabilize the operating wavelength of the semiconductor laser either to a single channel wavelength, or they can allow the laser to hop between operational channel wavelengths.
Lockers have been proposed that are based on etalons. Typically, the thickness of the etalons must be controlled to define the locker's frequency locking range and the spacing of the frequency locked channels, which is given by the etalon free spectral range (FSR). Further, the etalons reflectivities must be controlled in order to achieve the desired amount of wavelength discrimination.